This invention relates generally to a method and apparatus for removing unstable cores from the earth and relates more particularly to a method and apparatus for recovering substantially intact cores of hydrocarbon hydrates from the earth, including those located underneath a body of land or underneath a body of water.
Methane and other hydrocarbons are known to react with liquid water or ice to form solid hydrocarbon hydrates. These compounds are believed to exist in very large quantities in Arctic regions in gas-bearing sediments which lie between about one thousand and a few thousand feet below the earth surface. Therefore, these hydrates represent an enormous potential resource of hydrocarbons. Additionally, it is believed that these hydrates probably form to some extent in water-injected oil wells; and, therefore, the occurrence of these hydrate deposits within the earth is an important topic of study. However, relatively intact hydrate cores must be recovered in order to study various properties of the formation, including permeability distribution, excess gas or water content, and the acoustic and thermal properties of hydrate-bearing sediments.
Conventional coring systems are inadequate for recovering such unstable cores. In a conventional coring system, a pipe which is open at the bottom is drilled down into a deposit; and the core is simply brought to the surface through the pipe. No attempt is made to pressurize the pipe. And, unless one uses a device which can bring the core up very quickly (such as a wireline retrievable coring assembly) a period of several hours will be needed in order to remove the cores from the drill pipe. During this long period of time, hydrocarbon hydrate cores would be unstable. This is extremely significant because unstabilized hydrocarbon hydrate cores could cause an explosion, due to the rapid dissociation of the hydrates into the hydrocarbon gas and water. The danger of using conventional coring systems to remove hydrocarbon hydrates is described in C. Bily et al., "Naturally Occuring Gas Hydrates in the Mackenzie Delta, N.W.T.," Bulletin of Canadian Petroleum Geology, vol. 22, no. 3, (September, 1974), pp. 340-352. And, even if the hydrate cores are removed from the earth quickly by use of a wireline retrievable assembly in a conventional coring system, the unstabilized cores will rapidly disintegrate at the surface and (when located within metal tubing) they could act like bombs.
A pressure core barrel is a fully enclosed and highly pressurized device which is used to contain and bring up cores and adhering drilling fluid at the same pressures which were present in the earth formation. In pressure coring, an inner core barrel is closed at the top and bottom, thus sealing the core (located within the inner core barrel) under bottom hole pressure. The sealed, pressurized inner core barrel is generally brought up to the surface along with an outer barrel.
Coring of hydrates with a pressure core-barrel is considerably safer than using a conventional system, described above, provided that certain precautions are taken. These precautions are described in the Bily article cited above and include using cool drilling mud and special equipment at the surface for controlling the temperature of the hydrate-filled core barrel while it is being depressurized (as in the Scripps procedure, described below).
As reported by C. E. Ward and A. R. Sinclair in A Study to Determine the Feasibility of Obtaining True Samples of Oil and Gas Reservoirs, BERC/RI-77/10 (October 1977), on page 43, the most commonly used procedure for preserving pressure cores is by freezing with dry ice. Any drilling fluids located between the inner and outer core barrels must be removed prior to freezing. As disclosed in that report, this can be done by slowly forcing a gelled material through the barrel until all of the drilling fluids have been displaced. The core barrel is then placed into an insulated container and carefully frozen with dry ice, the freezing procedure requiring approximately one hour. Then, the pressure maintenance is disconnected and the outer core barrel is removed from around the inner core barrel. Standard pipe cutters are then used to section the inner barrel into easily handled lengths, the core is broken at each cut, and each section is wrapped with aluminum foil or plastic as quickly as possible (in order to prevent prolonged exposure of the core to materials such as nitrogen, hydrogen, and oxygen which can alter formation wettabilities), then replaced into insulated containers, and then covered with dry ice. After arrival at the laboratory, (as disclosed on p. 45 of that report), in order to prepare the frozen cores for analysis, the sleeve (i.e., barrel) containing the frozen core is removed by milling two grooves down opposite sides of the core, while maintaining the core in a frozen state, cooling the barrel with liquid nitrogen during the milling operation. Then, the sleeve is removed by prying it loose from the core. However, this method is not suitable for stabilizing hydrocarbon hydrates because much lower temperatures must be used to safely stabilize hydrate cores. Methane gas trapped inside the core barrel could still exist at high pressures (several hundred psi) at dry ice temperature. In addition, this method is very expensive and utilizes a lot of bulky, complicated equipment which would be very difficult to transport and use in the remote Arctic or offshore environments where hydrates have been encountered. In this method the drill string is pulled out after every core, and thus the system is not usable for operations like the Deep Sea Drilling Project (DSDP) being conducted by the Scripps Institute. Furthermore, any handling of unstabilized hydrates is very dangerous.
In order to retrieve hydrocarbon hydrates cores, Scripps Institute has designed special pressure coring equipment. This is desirable in cases where the drill string cannot be pulled out of the hole after coring, and the Scripps equipment is described in "DSDP Develops New Coring System," Petroleum Engineer International, February, 1981, p. 12. Only the inner core barrel is brought up to the surface; and it is wireline retrievable. However, the pressure core barrel equipment is very expensive (and, thus, is non-expendable), and is complicated (including a number of valves and seals). And to date, the only information about cores which has been obtained by using that pressure core barrel is blowdown information obtained by slowly bleeding off the hydrocarbon gas. No intact cores have been obtained by using this type of equipment. Instead, only various parameters were calculated from measurements on the gas that was given off when the hydrates dissociated. If the hydrate core was stabilized by freezing the core inside the pressure core barrel, the drilling fluid surrounding the core would also freeze. This drilling fluid, which is always present with the hydrate cores, would form an extremely tight fit in the pipe. These cores cannot simply be melted out of the pipe because the pipe and hydrate form a system which could generate high pressures and is thus very dangerous. Thus, the core barrel would have to be cut apart (and destroyed) to recover the core.
Therefore, despite what has been known in the prior art, there has been until now no method and apparatus for retrieving substantially intact cores of hydrates.